Process for Preparing Shaped Particles From Rehydratable Alumina

The invention relates to the preparation of shaped alumina particles. In particular, it relates to forming shaped particles through utilization of a heated shaping medium. More particularly, it relates to an oil drop method for producing spherical alumina bodies from an alumina composition containing a substantial portion of a rehydratable form of alumina.

Various processes for preparing spherical aluminas previously have been proposed. One such prior method for producing such spherical shapes incorporates an oil drop procedure whereby drops of an aqueous slurry of alumina are dispersed into a water-immiscible suspending medium. Though seemingly simplistic in theory, the oil drop method has presented considerable practical operational problems. The prior art methods, exemplified by U.S. Pat. Nos. 2,620,314 and 3,558,508, primarily are directed to the use of an aqueous slurry of an alumina sol including various chemical agents, so that the sol will set to a gel within the time interval of spherical particle formation, while the alumina drops are passing through the column of water-immiscible liquid. Hence, the selection of the starting alumina composition is critical in order to obtain firmly formed particles that, in addition, will not dissolve or crack during later processing or use.

The present invention is directed to the use of a novel alumina composition in an oil drop formation method. Unlike the prior art advancements in this field, the present invention does not incorporate the use of outside chemical reagents to induce rheological changes; rather, the novelty rests in the alumina composition itself which is capable of forming firm spherical shapes independently through its own internal chemistry.

According to the principles of the present invention there is provided, in a method for producing shaped alumina particles suitable for use as catalysts and catalyst supports comprising preparing an aqueous slurry of an alumina composition containing a substantial portion of a rehydratable alumina, shaping the alumina into desired form, rehydrating to the shaped alumina and curing and calcining the shaped alumina particles to produce catalyst and catalyst support material, an improvement comprising introducing said aqueous slurry of a rehydratable alumina composition to a shaping medium whereby the alumina is fashioned into a desired configuration, and applying heat to said shaping medium to rehydrate and harden the alumina while it is being subjected to the influence of the shaping medium.

A preferred method for producing spherical alumina particles according to the invention uses an alumina composition including a substantial portion of a rehydratable alumina form. Such an alumina composition can be produced by flash calcining hydrated alumina, such as is generally made from bauxite ore using the Bayer process, to form a partially dehydrated, rehydratable product consisting of anhydrous alumina, lower hydrate forms of alumina, alumina monohydrate, and unreacted trihydrate. The rehydratable alumina composition which can be used in the present process may vary in composition. The rehydratable powder commonly can be characterized by its LOI (loss on ignition) and its RI (rehydration index). The LOI is determined by measurement of the amount of weight loss on heating the alumina powder at 1800.degree. F. for 2 hours. The RI, which denotes the amount of rehydratable alumina present in the powder, is computed according to the formula: ##EQU1## In general, the preparation method for the rehydratable alumina consists of partially dehydrating alumina trihydrate by passing it through a flow of high temperature gas for a fraction of a second to several seconds. The composition of the resulting product varies according to the trihydrate feed rate, the particle size, the gas temperature, and the residence time of the particle in the gas stream. This rehydratable powder composition can be milled or ground to reduce the particle size and then mixed with water and formed. In the rehydratable alumina composition of the present invention LOI measurements in the general range of 3-15 are considered preferable. The rehydration index of the powder should be between 15 and 80, with a preferred range of 40 to 80. The formed alumina may be hardened or cured to increase crush strength. Steam or hot water treatment curing has been found to be preferable. The alumina forms are then dried and calcined.

A key feature of the rehydratable alumina composition is that an aqueous slurry of the composition requires no additional chemical reagents in order to harden into an isotropic solid while passing through the immiscible phase. A slurry with a solids content of about 50-60% has been found to be of a preferred consistency. Mere application of heat to the alumina in the presence of water serves to effectively rehydrate the rehydratable alumina form and accordingly harden the alumina sphere into a firm discrete particle. Temperatures in the range of about 80.degree.-100.degree. C. have been found to be preferable. The surface tension forces in the immiscible phase form the slurry drops into spheres, while, at the same time, the alumina is firmed by internal rehydration. An important aspect of the process is to control the droplet buoyancy and oil viscosity so that critical hardness is achieved while the drops are in free fall. Any suitable water-immiscible liquid which does not vaporize at the rehydration temperatures may be employed. While it is possible to use immiscible liquid which has a higher density than that of the formed alumina, it is preferred to use a liquid of lower density so that the formed alumina will free-fall to the bottom of the forming column rather than rise to the top thereof. A blend of polyterpene resin and mineral oil has been determined to be a suitable immiscible phase of acceptable viscosity and density.

Spherical shaped alumina offers certain advantages and hence is preferred in various catalyst applications. The lack of sharp edges on the spheres reduces wear and handling problems. Also the spherical shape often permits more uniform packing and thereby reduces channeling tendencies in a reaction zone.

By utilizing the ability of the rehydratable alumina slurry to harden directly into a formed shape, a distinct energy saving advantage is obtained over other practiced forming techniques. For example, the energy intensive steps of mulling and extruding, essential steps in the formation of extrudates, can be totally obviated. Further, the nature of the forming process of the present invention yields a more nearly isotropic solid alumina particle which minimizes planes of weakness, rendering catalyst and catalyst support particles with higher crush strength and porosity equivalent to extrudates.

Low density alumina catalyst substrates may be made using the present process by incorporating a combustible filler in with the alumina plus water when preparing the aqueous slurry. The combustible filler is destroyed during the calcination subsequent to the shaping process so as to yield a low density product. By low density is meant a compacted bulk density in the order of 12 to 32, preferably 20-30, and most preferably 26-30 lbs./cu. ft.

Suitable combustible fillers include such as microcrystalline cellulose, starch, wood flour, fine particle carbon black, and saw dust. Preferably the filler is microcrystalline cellulose. The fillers are generally used in amounts of from about 2 to 25 percent by weight of the dry ingredients, preferably 5 to 20 percent. The solids content of the slurry should preferably be in the range of about 25-50%.

The microcrystalline cellulose usable herein is purified, partially depolymerized cellulose prepared by treating alpha cellulose, obtained as a pulp from fibrous plant material, with mineral acids. It may be prepared as disclosed in U.S. Pat. No. 2,987,446 and is available under the tradename Avicel.RTM. of FMC Corp. Alternatively, depending upon the availability of materials, it may be used without the drying operation specified in the patent.

The following examples demonstrate preparation of catalysts and catalyst supports using the method of the present invention. They are not intended to be limiting but merely illustrative.

A suitable aqueous alumina slurry with a solids content of 50-60 can be prepared through a method including the utilization of an ion exchange resin for removing ionic impurities from the alumina prior to forming. Such a slurry was prepared by stirring 1262 g of a rehydratable alumina powder composition (LOI 9.7 RI 41, 0.15% Na.sub.2 O) into 1000 ml of ice chips to which water was added to fill the interstices. This resulted in a thick slurry at a temperature of 10.degree. C. To the chilled slurry was introduced 100 ml of an ion exchange resin (Dowex 50 W-X 8, a sulfonated styrene divinyl benzene polymer) with a particle size of 20-50 mesh. An exotherm occurs as the resin removes ionic impurities and the slurry becomes less viscous. The pH of the slurry fell from 10.2 to 7.9 in 4 hours, accompanied by a temperature rise to about 20.degree. C. The slurry, now quite fluid, was separated from the used ion exchange resin by screening the resin off on a 50 mesh screen. The ion exchanged alumina was analyzed at 0.03% Na.sub.2 O. The resultant aqueous alumina slurry has a solids content of 50.3%.

An aqueous alumina slurry prepared as described in the procedure set forth above (solids content 50.3%) was observed to undergo rapid setting upon application of heat. In order to form the alumina into beaded shapes, droplets of the slurry formed from a small orifice 1.5 mm in diameter were dropped into an oil bath. The surface tension forces in the immiscible oil phase forms the slurry drops into spheres of the alumina passes through the oil bath. In order to rehydrate and harden the alumina as the beaded shapes are passing through the oil, the oil was heated to 80.degree.-100.degree. C. Critical hardness was achieved while the drops are suspended in free fall through the oil, so that sufficient strength developed before the drops reached the bottom of the oil container where they would otherwise tend to flatten out on impact. A blend of 80% poly terpene resin and 20% mineral oil was used as the immiscible phase. Droplet fall times in a 2-3" oil layer were in the 3-60 second range; this was quite sufficient for the development of firm formed beads capable of maintaining their shape. The beads then were cured in the oil for about another additional 3 hours. A catalyst support was made from these beads by subjecting them to drying and calcining treatments. The finished product exhibited the following properties:

An alternative approach to utilizing the present inventive concept accommodates the formation of extrudates. Instead of shaping beads through use of the surface tension forces in an immiscible phase, cylinders (or other preferred shapes) can be formed by forcing an aqueous alumina slurry through tubes. Application of heat to the tubes serves to rehydrate and harden the alumina as it is being formed. Accordingly, the shaped alumina is ejected from the tubes as rigid cylinders. The following example illustrates this embodiment of the invention.

An aqueous alumina slurry was prepared according to the procedure set forth above. This slurry then was pumped by a Masterflex Variable Speed Tubing Pump, equipped with 0.02 "ID vinyl tubing. A tubing transition was made to thin wall polytetrafluoroethylene tubing of 1/16" ID. The polytetrafluoroethylene tubing was arranged to extend one foot into an enclosure heated by atmospheric pressure steam. As the alumina slurry was pumped through this heated extrusion die, the heat caused it to harden inside the die and be ejected as a rigid cylinder. The pump rate was adjusted so as to produce cylinders of desired firmness upon leaving the die. Upon emerging from the die, the extrudates fell into a pool of 100.degree. C. steam condensate at the bottom of the steam enclosure, where they were allowed to cure.

The basic procedure of Example I was repeated to make low density beads as below. The aqueous alumina slurry was diluted to 40% solids by the addition of 25.8 g of water to 100 g of the 50.3% solids slurry. As the dilution was made the alumina tended to settle into a dense cake with a supernatant water layer. At this point, 1 g of microcrystalline cellulose (Avicel.RTM. of FMC Corp.) was added with stirring. The settling was inhibited and beads were formed as in Example I.

The beads were calcined, and pore volume measured by water titration was 1.07 ml/g.

The compacted bulk density at 0.38 void fraction was 28.1 lbs./cu. ft.

The procedure of Example III was repeated except the solids content was reduced to 30% and the amount of microcrystalline cellulose was increased to 1.35 g.

The pore volume of the resultant beads was 1.65 ml./g. The compacted bulk density at 0.38 void fraction was 20.0 lbs./cu. ft.

Low density alumina beads were prepared by dispersing 20 g of microcrystalline cellulose in 60 g of water with stirring. This mixture was then mixed with 111 g of a rehydratable alumina powder and 30 g of ice. The pH of the resultant slurry was 9.61. 20 drops of 70% HNO.sub.3 were added and the pH was reduced to 7.48.

Then the beading procedure of Example I was used on the above-prepared slurry.

The resultant beads had a compacted bulk density of 13.1 lbs./cu. ft.